The “Rayleigh diffraction limit”, which is the spatial resolution with which an object can be detected, is limited by the wavelength of the radiation used for detection. Higher frequencies are therefore required to resolve smaller objects.
Microwaves of high frequency are absorbed in the atmosphere at rates exponentially higher than microwaves of low frequency. Accordingly, low frequency radar is preferred for longer range. However, due to the Rayleigh diffraction limit, the ability to distinguish two objects adjacent to each other, referred to as “resolving power”, is proportional to the ratio of wave length to aperture. As a result, for a unit aperture, radar can only distinguish an object if the wavelength of the electromagnetic radiation is the same or smaller than the object. The Rayleigh diffraction limit combined with the earth atmosphere's attenuation profile forces radar designers to choose between long range at low resolution, or short range at high resolution. In an extreme example, penetrating radars such as foliage penetrating radar (FOPEN) or ground penetrating radar (GPR) require low frequencies to minimize attenuation within the penetrated medium. Consequently, only the very largest objects can be resolved, diminishing the utility of such radar systems.
According to concepts of quantum mechanics, a quantum system may exist in several states simultaneously corresponding to different values of a physical observable such as position, momentum, or spin. Changes among properties of entangled photons are correlated. The composite system is described by a nonseparable state, that is, a superposition of substates describing eigenstates of the specific observables. Each of these substates corresponds to eigenvalues of some set of observables (e.g., particles' positions). An entangled state can thus be put in different forms, each being adapted to the analysis of a specific detection procedure. In quantum entanglement, the quantum states of two or more particles are described with reference to each other, even though the individual objects may be spatially separated.